Dual rail IC + TLE2426 needs -- Two bypass caps, or just one?

Thread Starter

SporkInTucson

Joined Dec 17, 2011
4
On another closed thread, SgtWookie wrote:
If the IC has a dual-rail supply (eg: Vcc/Vdd, GND, and Vee/Vss) then you need at least two caps; one from GND to Vcc/Vdd and one from GND to Vee/Vss.
I wonder if this is true in the particular case of a dual rail op amp being powered by a TLE2426 rail splitter which isn't powering anything else. If I have a bypass cap across the TLE2426 IN and COMMON, that "stiffens" the voltage into the TLE. From there, I assume that I can depend on the TLE to reliably divide that stable difference into halves (or as close to halves as that particular TLE can manage), without introducing significant new noise. If I'm reading the data sheet correctly, the noise out of the TLE2426 is about 120 microvolts, since I'm using the three lead version which has no noise reduction pin.

Is that 120 microvolts of noise enough that I should forget about the cap across IN and COMMON, and just put one each on the op amp's V+ and V-? Are they going to make much of a difference with respect to noise at that level?
 

GopherT

Joined Nov 23, 2012
8,012
You should still have capacitors across the power supply pins of the op amp (always a good practice). Also, if you are using a ground pin as a voltage reference to the op amp, then two capacitors are best (Vss to ground and ground to Vdd). Fast switching of digital components downstream of your power controller can cause noise in the circuit that the controller overcome - especially as you approach the current limit of the controller.
 

Thread Starter

SporkInTucson

Joined Dec 17, 2011
4
You should still have capacitors across the power supply pins of the op amp (always a good practice). Also, if you are using a ground pin as a voltage reference to the op amp, then two capacitors are best (Vss to ground and ground to Vdd).
Hmm... I'm not sure how the terms "Vss, Vdd, and ground" map to my case. Here's a simplified diagram:



Not shown in this diagram, I currently have one cap across V+ and V- on the daughter board, as described in my original post. If I were to have two, which would make more sense for the daughter board?

  • From VGND to V- and from V+ to V- (with V- as the ground plane)
  • From V- to VGND and from V+ to VGND (with VGND as the ground plane)
  • From V- to VGND and from V+ to VGND (with V- as the ground plane)
  • None of the above

Is the decision about where capacitors go completely independent of the choice for ground plane? Should bypass caps always bypass to the ground plane if one exists?

Thanks for your feedback, Gopher.
 
Last edited:

MrChips

Joined Oct 2, 2009
25,004
None of the above.

Your PCB Ground Plane is V- on both boards.

Cap from V+ to VGND.
Cap from VGND to V-.

VGND is a reference, not a GROUND PLANE.
 
Last edited:

Thread Starter

SporkInTucson

Joined Dec 17, 2011
4
Thanks, MrChips. You must have been replying with "none of the above" and your alternative, just as I was editing my post to add the same as a third option.
 
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